A NASA Hubble Space Telescope image of a spiral-shaped disk of hot gas in the core of active galaxy M87. HST measurements show the disk is rotating so rapidly it contains a massive black hole at its hub.

A black hole is an object that is so massive yet compact nothing can escape its gravitational pull, not even light. The object at the center of M87 fits that description. It weights as much as three billion suns, but is concentrated into a space no larger than our solar system.

Now that astronomers have seen the signature of the tremendous gravitational field at the center of M87, it is clear that the region contains only a fraction of the number of stars that would be necessary to create such a powerful attraction.

The giant elliptical galaxy M87 is located 50 million light-years away in the constellation Virgo. Earlier observations suggested the black hole was present, but were not decisive. A brilliant jet of high- speed electrons that emits from the nucleus (diagonal line across image) is believed to be produced by the black hole "engine."